There cannot be obtained diene polymers having a superior balance between a heat resistance and workability, by means of polymerizing non-conjugated diene compounds such as 1,6-heptadiene represented by the hereinafter-mentioned formula (3) having two carbon-to-carbon double bonds, in the presence of a commonly-used addition polymerization catalyst. The reason is that one of the two carbon-to-carbon double bonds is involved in not an addition polymerization reaction but an undesirable reaction such as a cross-linking reaction, although another carbon-to-carbon double bond is involved in an addition polymerization reaction, and as a result, there is obtained only a polymer, which (i) has no cyclic structure in its main chain contributing to a heat resistance, and (ii) is not necessarily a thermoplastic polymer contributing to workability.
On the other hand, an ethylene-cyclopentane alternating copolymer, which is obtained by coordination ring-opening polymerization, and is disclosed in Macromolecules, Vol. 35, pages 9640-9647 (2002), has a problem in that (i) the copolymer needs to be molded at a high temperature of 200° C. or higher, because of its high meting point of 180° C. or higher, and (ii) a polymer product of coordination ring-opening polymerization needs to be further hydrogenated, in order to obtain the ethylene-cyclopentane alternating copolymer, although the copolymer has a cyclic structure in its main chain contributing to a heat resistance.